Direction crossing detector for containment boundary

ABSTRACT

A containment area can be defined by a single cable carrying an asymmetric electromagnetic signal that generates a magnetic field comprising an asymmetric waveform. A single inductor circuit configured to detect a single axis of the magnetic field can detect the asymmetric waveform and determine which direction the inductor is traveling relative to the cable. A human-propelled cart can have a wheel that includes the single inductor circuit and detect whether the cart is being pushed from inside-to-outside the containment area (which may reflect the cart is being stolen or improperly used) or from outside-to-inside (which may reflect the cart is being returned). The cart can include an anti-theft system (e.g., a locking or braking wheel), which can be triggered if the cart is being moved from inside to outside the containment area. The single cable, single inductor system can be less expensive and more efficient than multi-cable, multi-inductor systems.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

Any and all applications for which a foreign or domestic priority claimis identified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57. Thepresent disclosure references various features of and claims priority toU.S. Provisional Pat. App. No. 62/374,677, Aug. 12, 2016. The entiredisclosure of U.S. Provisional Pat. App. No. 62/374,677 is hereby madepart of this specification as if set forth fully herein and isincorporated by reference herein for all purposes, for all that itcontains.

BACKGROUND Field

This application relates to the detection of electromagnetic fields andto the tracking, detection, or loss prevention of non-motorized,human-propelled carts, including but not limited to shopping carts.

Description of the Related Art

A variety of commercially available cart containment systems exist fordeterring the theft of shopping carts from a retail store. Typically,these systems include one or more cables embedded in the pavement of astore parking lot to define an outer boundary of an area in whichshopping cart use is permitted. The cables carry an electromagneticsignal that can be detected by electromagnetic sensors in the cart(typically in the wheel). If the cart is pushed across the embeddedcable, the electromagnetic signal is detected, and a cart anti-theftsystem can be actuated (e.g., a brake in a wheel can be actuated toinhibit movement of the cart).

SUMMARY

In an illustrative, example system, a containment area can be defined bya single cable carrying an asymmetric electromagnetic signal thatgenerates a magnetic field comprising an asymmetric, time-fluctuatingwaveform (typically at very low frequencies below about 9 kHz). A singleinductor circuit configured to detect a single axis of the vectormagnetic field can detect the asymmetric waveform and determine whichdirection the inductor is traveling relative to the cable. Anon-motorized, human-propelled cart can have a wheel that includes thesingle inductor circuit and detect whether the cart is being pushed frominside-to-outside the containment area (which may reflect the cart isbeing stolen or improperly used) or from outside-to-inside (which mayreflect the cart is being returned). The cart can include an anti-theftsystem (e.g., a locking or braking wheel), which can be triggered if thecart is being moved from inside to outside the containment area. Thesingle cable, single inductor system can be less expensive and moreefficient than multi-cable, multi-inductor systems.

Details of one or more implementations of the subject matter describedin this specification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings, and the claims. Neitherthis summary nor the following detailed description purports to defineor limit the scope of the inventive subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an example of a retail installation ofa cart containment system.

FIG. 2 schematically shows an example of a system including a shoppingcart wheel moving outward toward the buried cable, prior to passing overthe buried cable.

FIG. 3A is a graph of an example waveform that can be driven through aburied cable.

FIG. 3B is a graph that shows an example waveform that is the sum of twosine waves one octave apart.

FIG. 4A shows an overall block diagram of an example of a singleinductor direction-detector receiver to detect a VLF signal from theburied cable.

FIG. 4B is a block diagram of a single inductor direction-detectorreceiver that is another version of the receiver shown in FIG. 4A.

FIG. 4C is a block diagram of a single inductor direction-detectorreceiver that is another version of the receivers shown in FIGS. 4A and4B and may be used with a ramp waveform

FIG. 5A shows an example schematic of a single inductordirection-detector receiver configured for dual sine wave detection witha tank circuit frequency shifter.

FIG. 5B shows another example schematic of a single inductor directiondetector receiver for detecting a ramp waveform.

FIG. 6 is a graph that shows an example of a 500 Hz saw tooth waveform.

FIGS. 7A-7D are graphs that show example waveforms under unsaturatedreceiver conditions and how they can be used to determine the directionof a cart. The graphs in FIGS. 7A and 7C are for the cart wheel on afirst side of the buried cable, and the graphs in FIGS. 7B and 7D arefor the cart wheel on the other side of the buried cable.

FIGS. 8A and 8B are graphs that show example waveforms under saturatedreceiver conditions and how they can be used to determine the directionof a cart.

Unless the context indicates otherwise, like reference numerals refer tolike elements in the drawings. The drawings are provided to illustrateembodiments of the disclosure described herein and not to limit thescope thereof.

DETAILED DESCRIPTION

Overview

Many shopping cart containment systems, such as the CartControl®containment system from Gatekeeper Systems Inc. (Irvine, Calif.), makeuse of a buried cable transmitting antenna to mark the boundary of theregion in which the shopping cart must be contained. If the cart crossesthe boundary (e.g., passes above the buried cable), an electronicapparatus in the shopping cart wheel decodes a modulated very lowfrequency (VLF, e.g., 8 kHz) radio frequency (RF) carrier which ispresent on the cable and activates a mechanism that inhibits the cart'smotion (e.g., by braking, locking, or inhibiting rotation of a wheel ofthe cart). Embodiments of such electronic apparatus are described inU.S. Pat. No. 6,127,927, Anti-theft Vehicle System, which is herebyincorporated by reference herein in its entirety.

At the system level, it may be desirable to be able to detect in whichdirection a cart is crossing the containment boundary: from inside tooutside (outgoing) or from outside to inside (incoming). An inside tooutside crossing typically calls for activating the motion-inhibitionmechanism (e.g., the cart is being stolen or moved into an unauthorizedarea); an incoming crossing (e.g. a locked cart being dragged backacross the boundary into an authorized area) typically calls forreleasing the motion-inhibition mechanism, so that the cart can again beused normally.

If the cart cannot detect the direction in which it is crossing theburied cable, one possible solution is to use two nested buried cables,e.g., the outer cable broadcasting “lock” and the inner cablebroadcasting “unlock”. Thus a cart crossing first the “lock” cable andthen then “unlock” cable knows that it is moving inward across thecontainment boundary and can unlock. However, this solution requiresinstallation of two separate cables, which may increase expense andcomplexity of certain such systems.

FIG. 1 schematically illustrates an example of a retail installation ofa cart containment system 100. FIG. 1 includes a parking lot 102, aburied cable 104 (illustrated by dashed line lines), a store 106 havingExit A and Exit B, a first transmitter TX#1, and a second transmitterTX#2. A current through the buried cable causes a magnetic field {rightarrow over (B)} to be generated. An induction circuit 108 can move overthe buried cable 104 in or out of the parking lot 102. As describedbelow, the induction circuit can be configured to measure a singlecomponent of the vector magnetic field {right arrow over (B)} generatedby the current in the buried cable. The induction circuit may compriseonly a single inductor having an axis along which the component of{right arrow over (B)} is measured. Also as further described below,such single induction circuit embodiments can be advantageous comparedto dual induction circuit embodiments configured to measure twocomponents of the vector magnetic field {right arrow over (B)}(typically components along two orthogonal directions).

The buried cable 104 forms a loop, with an area inside the loop. Theburied cables carry a VLF (typically less than 9 kHz) electromagneticsignal. The electromagnetic signal can encode a motion-inhibitioninstruction that can trigger a motion-inhibition system in the carts(e.g., by locking, braking, or inhibiting rotation of one or more cartwheels). In this particular example, there are two exits from a store.Exit A leads to a parking lot, and Exit B leads to an interior area of amulti-store shopping mall. A desired system behavior may be thatshopping carts leaving the store through Exit A are free to roam theparking lot, but their motion will be inhibited if the person pushingthe cart attempts to take the cart out of the boundary of the parkinglot (where a VLF cable is buried). Similarly in this example, theshopping carts are for the use of patrons of the store only, not for useelsewhere in the shopping mall, so carts that attempt to cross theburied cable just inside Exit B will also have their motion inhibited.The return loop of the buried cable at Exit B may be out of the plane ofthe floor (e.g. the cable return may be routed through the frame of ExitB).

In this example, there are two separate VLF (e.g., 8 kHz) transmitters,one for each loop of cable (e.g., TX #1 and TX #2). Each transmitter canbe configured to transmit a VLF signal across a burred wire. In someembodiments, one VLF transmitter can transmit VLF signals throughmultiple loops of cables. A receiver in a cart can detect the VLF signalto determine the cart's proximity to the buried cable 104.

As the VLF transmitter TX #1 transmits a VLF signal through the buriedcable 104, a current is driven through the buried cable 104 in theclockwise direction as shown in FIG. 1. According to the right hand rulefor current carrying wires, the current through the buried cable 104generates the magnetic field {right arrow over (B)} around the buriedcable 104 in the directions shown in FIG. 1. In some embodiments, thecable currents can be kept under 1 ampere root-mean-square (RMS), under200 mA RMS, about 120 mA RMS, under 100 mA RMS, or other value, or theinduced magnetic field can couple into ferromagnetic structures (e.g.,buried cast iron pipes or electrical conduits) and cause poor orunreliable system performance.

Further details regarding cart containment systems (that are usable withembodiments of the present disclosure) are described in U.S. Pat. No.8,463,540, which is hereby incorporated by reference herein in itsentirety for all it discloses.

Additionally, although examples described herein are in the context ofshopping carts in a retail environment, this is for purpose ofillustration and is not intended to be limiting. The disclosedembodiments can be used in other contexts where containment of wheeledcarts to an authorized area is desired such as, e.g., warehouses andwarehouse carts, transportation depots (e.g., airports, train stations,bus stations, etc.) and luggage carts or baggage carts, medicalfacilities (e.g., hospitals, doctors offices, nursing homes,convalescent or treatment centers, etc.) and wheelchairs, hospitalbends, medical device carts. Carts include any type of non-motorized,human-propelled cart. Such carts can be equipped with motion-inhibitionsystems, which may take the form of a brake, lock, or device to inhibitwheel rotation associated with one or more cart wheels. For example, acart wheel can include an embodiment of the wheel brake described inU.S. Pat. No. 8,463,540. The motion-inhibition system may communicateelectromagnetically with the buried cable (e.g., wirelessly, to receivea VLF signal from a cable).

Example Cart Containment Implementations

A low power consumption 8 kHz receiver suitable for receiving anddecoding the VLF signals of a lock line can be found in a SmartWheel®cart wheel available from Gatekeeper Systems, Inc. (Irvine, Calif.).This receiver uses an inductor as both its antenna and as the inductiveelement of the receiver's resonant tank circuit, which can be tuned todetect the frequency of the VLF signal propagating in the buried cable104.

Determining which direction a cart is moving relative to a cable isstraightforward to solve given two inductors, with axes mounted at 90degrees to each other, where the angle of each inductor to thehorizontal is known. The angle can be known either by mechanicalconstraint, or by including an angle sensor such as amicroelectromechanical systems (MEMS) accelerometer which co-rotateswith the inductors. An example of a two inductor solution to the problemcan be found in U.S. Pat. No. 8,749,385 to Bernhard, et al. That patentrefers to “magnetic field sensors” and an “AC magnetic field” withoutusing the word “inductor”.

The inductor used to form an appropriate VLF antenna may be relativelybulky (e.g., 7 mm diameter by 12 mm length), and a dual inductorreceiver may require substantial redesign of the wheel's internalmechanical components (e.g., to fit the line crossing detector, RFtransceivers, wheel brakes, and other components inside the wheel),especially if two inductors are aligned on two different axes.

Various circuits and devices other than inductors can detect alternatingcurrent (AC) magnetic fields in the kHz range. For example,magnetoresistive (MR) sensors from such suppliers as Sensitec (Mainz,Germany) can detect time-varying (e.g., kHz) magnetic fields and may beused in certain embodiments of the circuits described herein. But size,cost, and power budget considerations tend to make inductors thepreferred magnetic field sensor for applications such as VLF receptionin shopping cart wheels.

In particular, to prolong battery life, an active idle current for theentire receiver circuit (e.g., when the receiver circuit is not beingexcited by the nearby presence of the buried cable) can be on the orderof 12 microamps or less, and a current when the receiver circuit isbeing excited by the VLF field can be on the order of 50 microamps orless (in both example cases the power supply to the receiver is assumedto be the voltage of a single cell lithium battery, e.g., no more thanabout 3 volts depending on the battery chemistry). This level of idlecurrent is difficult to achieve with MR sensors.

It may be desirable to be able to detect a line crossing using a singleinductor. For example, it can be desirable to determine whether a movingcart is exiting or entering a perimeter without the expense of burying asecond cable around the perimeter, without the expense of mounting asecond inductor in a wheel, without having component area internal tothe wheel occupied by the second inductor, or without having powerconsumed by the second inductor. In some embodiments disclosed herein,the direction of a moving cart (e.g., entering or exiting) can bedetected for a perimeter having one buried cable using a detectionsystem that includes one magnetic field sensor (e.g., one inductoraligned on a first axis) without including a second sensor (e.g.,without a second inductor aligned on a second axis different from thefirst axis).

Also, embodiments of a single inductor receiver as described herein havea somewhat lower manufacturing cost for multiple reasons (e.g., fewercomponents with lower component cost, and also fewer specializedmanufacturing operations, since the receiver inductor may be a throughhole part which requires hand soldering and mechanical support and useof a single inductor, rather than two inductors, reduces the amount ofhand soldering or mechanical support needed).

Examples of qualities of an inductor that may be beneficial to a singleinductor VLF detection crossing detector include one or more of thefollowing. (1) The inductor should have sufficient inductance togenerate a reliably detectable voltage (EMF) in response to otherwiseacceptable magnetic field strengths induced by the VLF signal from theburied cable 104. A typical usable inductance value is on the order of afew millihenries (mH) to tens of millihenries (e.g., 1 mH to 50 mH).Higher EMFs associated with larger inductances generally improve thesensitivity and signal-to-noise (SNR) of the receiver, which must bebalanced against the larger size and greater cost of the largerinductor. (2) The inductor should have mechanical stability anddurability against shocks. In the case of a shopping cart wheel, shocksin excess of 2000 g (where g is the Earth's gravitational acceleration)are not especially uncommon. Some inductor materials can last longerthan others. For example, ferrite core inductors generally cannotrepeatedly tolerate that sort of shock level for prolonged periods oftime. (3) The inductor should have smaller changes in inductance as aresult of mechanical vibration in either the VLF passband (less than 9kHz, e.g., 8 kHz) or the modulating frequency (typically a few hundredHz) of the VLF cable signal. Shopping cart wheels also sometimes vibratewhile rolling (e.g., over rough parking lot surfaces) and changes in theinductance due to the vibration of the shopping cart can cause spurioussignals at the receiver. The foregoing are several desirable qualitiesof an inductor but are not intended to be requirements for anyparticular inductor.

Example Fluctuating Magnetic Field Sensors

Magnetic field sensors include inductors, magnetoresisitive sensors, andother circuits configured to be responsive to a fluctuating (e.g.,time-varying) magnetic field in a particular direction. In someexamples, such a time-varying magnetic field may be referred to as an ACmagnetic field, because such time-varying magnetic fields are commonlygenerated by AC circuits. However, this is for illustration, and an ACmagnetic field may refer to the fluctuating magnetic field generated bya DC current that includes a fluctuating part (e.g., a fluctuatingcurrent plus a larger DC offset). As is clear from Faraday's law, theEMF generated in an inductor is related to the time-varying magneticflux, and a DC offset current (if used) will not generate an EMF (orwill generate an EMF very much smaller than due to the time-varyingcomponent at typically kHz frequencies). As an example, an inductor cancomprise a coil of wire around a core that is sensitive to the componentof the magnetic field that is parallel to the coil axis. The terminductor, as used herein, is intended to refer to an inductive elementhaving inductance as the primary property for which the element is used.For example, a wire may have an undesirable parasitic inductance, but ifthe wire is used primarily to conduct electricity, then wire's primaryproperty is conductivity, and the wire is not considered an inductor.

As described herein, embodiments of the direction detection circuits arereferred to, for simplicity, as single inductor circuits, because thedirection detection capability of the circuit is provided by an ACmagnetic sensor that is configured to measure a single component of thevector AC magnetic field generated by an RF-frequency electromagneticcurrent in the cable 104 (e.g., a component of the vector AC magneticfield along a single axis). Such embodiments are simpler thantwo-inductor direction detection circuits, which typically use twodistinct inductors (often disposed at right angles to each other) tomeasure two separate components of the vector AC magnetic field.

The words single inductor are intended to refer to an induction circuitconfigured to measure a single component of the vector AC magnetic fieldalong a single axis in space. Such a single inductor often is just asingle, unitary inductor. However, in other implementations, such asingle inductor can comprise two or more inductors in series or inparallel that are configured to measure a single component of themagnetic field along a single axis in space. For example, two inductorselectrically connected in series and disposed along a single axis arethe electrical equivalent of a single inductor disposed along the singleaxis having a single inductance that is the sum of the inductance of thetwo inductors, and these two inductors function as a single inductor ifsubstituted for the one inductor. As another example, two inductorselectrically connected in parallel and disposed along a single axis (orparallel to the single axis) are the electrical equivalent of a singleinductor having a single inductance that is the reciprocal of the sum ofthe reciprocal of each inductance of the two inductors, and these twoinductors function as a single inductor if substituted for the oneinductor.

Example Operation of a Single Inductor Circuit for Direction Detection

In the example implementation shown in FIG. 1, the current in the buriedcable 104 surrounding the parking lot flows in a clockwise direction. Asfurther described below, in various implementations, the current may bea direct current (DC) that includes a DC offset plus a fluctuatingcurrent component (e.g., time-varying at VLF frequencies) that generatesa fluctuating magnetic field. Thus by the right hand rule, thehorizontal component of the magnetic field {right arrow over (B)} abovethe buried cable points inward, toward the parking lot, where cartmotion is authorized. In other embodiments, the current may be analternating current (AC).

The EMF, V, induced in an inductor with inductance L is the inductancemultiplied by the rate of change of the current I as expressed byV=L(dI/dt). An inductor 108 with a horizontal axis which is passing overthe buried cable will produce an EMF of the same sign as dI/dt in thecable (where I is the cable current), if the measured end of theinductor is pointing inward, and the EMF will be of the opposite sign asdI/dt if the inductor is pointing outward.

In practice and as further described below with reference to FIG. 2,because the desired detection point of a cart moving outward may be at aposition before the cart wheel passes directly over the cable, theinductor axis can be angled at an angle θ relative to the vertical toalign the axis of the inductor with the magnetic field vector at thatposition. Aligning the inductor so that it is substantially parallel tothe magnetic field vector {right arrow over (B)} (at the desireddetection point) improves the sensitivity of the detection circuitbecause the single component of the magnetic field detected by theinductor is approximately equal to the full magnitude of magnetic field.

FIG. 2 schematically shows a system 200 including a shopping cart wheel202 moving outward toward the buried cable, prior to passing over theburied cable 104. An inductor 204 is included inside the wheel anddisposed toward the leading edge of the wheel (e.g., the side of thewheel in the direction of motion of the cart). The wheel 202 can beconfigured such that the inductor 204 does not rotate with the rotationof the wheel, for example, by attaching the inductor to a chassis orcircuit board that is rigidly attached to a non-rotating axle asdescribed in, e.g., U.S. Pat. No. 8,820,447, which is herebyincorporated by reference herein in its entirety. The wheel 202 caninclude an angle detection device 230, which will be described below.

The buried cable can be buried by a first vertical distance 204 below asurface. The first vertical distance is typically one to a few cm. Atthe desired detection point, the inductor 204 can be a horizontaldistance 206 away from the buried cable 104 and a second verticaldistance 208 above the buried cable 104.

In this example, the cable current I is into the page so that directlyover the buried cable 104, the horizontal magnetic field component abovethe buried cable 104 points to the right (opposite the arrow showing theoutward motion of the wheel 202). As can be seen from FIG. 2, themagnetic field vector {right arrow over (B)} at the position of thewheel is angled downward as shown by arrow 210 (according to the righthand rule). Accordingly, as explained above, an inductor axis that isaligned at the angle θ relative to vertical will more closely align thedetection axis of the inductor with the cable's magnetic field. Theangle θ can be determined from the formula tangent θ equals the verticaldistance 208 divided by the horizontal distance 206.

The detection point can be selected so that the horizontal distance 206is close enough to the cable 104 such that the VLF signal has enoughpower to reach the inductor to cause a reliably measureable EMF and alsobe far enough to provide advance notice of a cart exiting a perimetersuch that a cart anti-theft system (e.g., a wheel braking system) hastime to respond and activate to inhibit motion of the cart. In someembodiments, the horizontal distance 206 can be about 0.25 meters, 0.5meters, 1 meter, 1.5 meters, 2 meters, etc. In some embodiments, theburied cable 104 is positioned at a distance 204 of about 1-3centimeters below the surface, and the inductor is positioned a fewcentimeters above the bottom of the wheel. Accordingly, in variousembodiments, the angle θ is in a range from 0 degrees to about 45degrees, 2 degrees to 30 degrees, or 5 degrees to 20 degrees.

In other implementations, the desired detection point might be rightover the buried cable 104 (e.g., the desired horizontal detectiondistance 206 is close to zero), and the magnetic field direction issubstantially horizontal. In such implementations, the inductor 204 maybe disposed in the wheel so that its detection axis is alsosubstantially horizontal, e.g., the angle θ is approximately 90 degrees.Accordingly, in various such embodiments, the angle θ is in a range from75 degrees to 125 degrees or 80 degrees to 110 degrees.

The buried cable 104 can carry a modulated signal that is encoded suchthat a positive slope of the current as a function of time (e.g., dI/dt)has a different magnitude than a negative slope, dI/dt. It is possibleto determine which direction the wheel 202 is crossing the buried cable104 by determining which direction of the EMF induced in the inductorhas a greater absolute magnitude. Thus, the signal modulated on thecable can be asymmetric under inversion and phase shifting (see, e.g.,the example waveforms described with reference to FIG. 3A, 3B, and FIG.6). Note that in many implementations, the current in the cable can bepositive or zero, such that the magnetic field points in a constantdirection. Accordingly, the waveform can have a direct current (DC)offset such that, when superimposed with an AC signal, the voltage (orcurrent) can remain positive with respect to local ground. Suchembodiments may simplify the transmitting amplifier design, because abipolar power supply is not needed. Further, the impedance of the cablemay be almost entirely reactive at VLF frequencies, so power expendedtransmitting the DC component in the cable is relatively small. Invarious implementations, a DC offset is used for shorter cables (e.g.,for containment boundaries in entrances/exits), whereas an AC signal(e.g., generated with a bipolar push-pull amplifier) may be used forlonger cables around a parking lot.

An example proposed single inductor direction detection circuit(described in detail below) provides a capability to determine thedirection that the inductor (e.g., in the wheel) crosses the cable. Thesingle inductor can be arranged along a single axis and configured tomeasure the magnetic field along that single axis. As described above,some embodiments feature one or more inductors on only one axis todetect the waveform in a buried cable 104. Some embodiments can includeinductors for other circuit functions (e.g., other than for detectingthe waveform in the buried cable 104).

A suitable waveform on the cable 104 can comprise a phased overlay oftwo frequencies in a 2:1 ratio, a 3:2 ratio, or any other ratio. In someembodiments, the 3:2 ratio is preferable, since the quality factor Q ofa receiver tank circuit can be somewhat higher (and thus the sensitivitygreater) with the signals being only half an octave apart, even thoughthe decoding may be slightly more complex. Further, the bandwidth of areceiver configured to detect 3:2 ratios is less than the bandwidth of areceiver configured to detect 2:1 ratios. In addition, ratios (such as3:2) that keep the transmitted electromagnetic signal (including thewaveforms described herein) below 9 kHz (e.g., in the portion of thefrequency spectrum unregulated by governmental agencies such as theFederal Communications Commission) can be advantageous. Otherembodiments can feature frequencies having any other ratio, such as 5:7,3:4, 3:5, 4:5, etc.

Example Single Inductor Direction Detection Waveforms

FIG. 3A is a graph 300 of an example waveform 306 that can be driventhrough a buried cable. The y-axis 302 indicates a voltage in mV, and anx-axis 304 indicates time in ms. The graph includes an example waveform306 and a second example waveform 308. The waveforms 306, 308 can be anasymmetric component as described below.

The example waveform 306 can is the sum of two sine waves, at 8 kHz and⅔*8 kHz, e.g., with frequencies having a ratio of 3:2:

${\sin( {{2\;{\pi( {\frac{2}{3}*8\; e\; 03} )}t} + {\pi/4}} )} + {{.6}\mspace{14mu}{\sin( {2\pi*8e\; 03*t} )}}$

The second waveform 308 is an inverted version of the first waveform306, e.g., the voltage V(waveform 308)=−V(waveform 306). Because thewaveform 306 comprises a plurality of sinusoidal frequencies (in thisexample, two with a 3:2 ratio), the waveform 308 cannot be inverted andphase-shifted (by any amount) to become a replica of the waveform 306.Accordingly, the example waveform 306 is asymmetric in time. Forexample, the waveform 306 and the second waveform 308 have differentsequences of local maxima and minima such that, even if the waveform 308is phase shifted and inverted (or proportionally scaled in voltage ifthe amplitudes were different), the phase shifted, inverted, and scaledwaveform 308 will be distinct from the waveform 306. An inductor in awheel that is approaching the buried cable from a first direction (say,from inside the parking lot) will measure one of the waveforms 306, 308,whereas the inductor will measure the other of the two waveforms 308,306 when approaching the buried cable from a second direction (oppositeto the first directions, say, from outside the parking lot). This occursbecause the measured end of the inductor points 180 degrees differentlyin these two scenarios. Accordingly, by broadcasting a signal comprisingan asymmetric waveform (such as shown in FIG. 3A or FIGS. 3B and 6described below) on the buried cable 104, a single inductor configuredto measure the magnetic field along a single direction can determinealong which of the first or second direction it is approaching thecable. Thus, the single inductor circuits described herein can determinedirection relative to a cable that generates a signal comprising anasymmetric waveform.

The reference point shown by dashed line 310 indicates an examplefeature of the waveforms that can be used to determine which waveform isdetected. For example, at 0.8 ms, the waveform 306 has a local voltagemaximum surrounded on either side by a local voltage minimum. Incontrast, the waveform 308 has a local voltage minimum surrounded oneither side by a local voltage maximum. These features can be detectedand distinguished, and the direction of the cart can be determined. Asdescribed below, other features of the waveform can be used as well, forexample, the relative location of peaks, valleys, positive slopes, ornegative slopes. As one example, the waveform 306 has a large magnitudevoltage maximum at about 0.66 ms followed by a negative slope prior tothe local peak at 0.8 ms.

Waveform features over a time period 312 can additionally oralternatively be used to distinguish the waveforms. The time period 312can be a fraction of the period of the waveform. For example, thereference point shown by dotted line 310 can be preceded by a voltagemaximum that that decreases (e.g., has negative slope) and wouldindicate that waveform 306 is detected or can be preceded by a voltageminimum that increases (e.g., has positive slope) and indicate that thesecond waveform 308 is detected. Accordingly, the direction of the cartcan be determined based on which waveform, 306 or 308 is detected by thesingle detection axis inductor 204.

FIG. 3B is a graph that shows another example waveform 356 that is thesum of two sine waves one octave apart. The ratio of frequencies in thisexample is 2:1 at 8 kHz and 4 kHz:sin(2π*4e03*t+π/4)+1.4 sin(2π*8e03*t)

The waveform 356 provides another example of waveform features that canbe detected by an inductor and will appear different based on thedirection of movement of the inductor. An inductor moving in onedirection can detect the waveform 356 (e.g., with local maxima around0.4V) while an inductor moving in the opposite direction will detect aninverted version of the waveform 356 (e.g., with local minima around−0.4V).

Example Single Inductor Direction Detection Circuits

FIG. 4A shows an overall block diagram of an example of a singleinductor direction-detector receiver 400 to detect the VLF signal fromthe buried cable 104. The receiver 400 includes a receiver tank circuit402 that includes inductor L1 and capacitor C1, with inductance andcapacitance chosen so that the resonant frequency of the tank circuit402 matches the frequency of the signal in the cable 104 (e.g., 8 kHz).The resonant frequency of the tank circuit can be 1/(2π√{square rootover (L1C1)}). The receiver also includes a receiver input amplifier404, a capacitor C2, a diode D1, a second receiver amplifier 406, afirst waveform feature identifier circuit 420, a second waveform featureidentifier circuit 422, and detection logic 416. The first waveformfeature identifier circuit 420 can include resistor R1, resistor R2,capacitor C3, comparator 408, and frequency counter 412. The secondwaveform feature identifier circuit 422 can include a resistor R3,resistor R4, capacitor C4, comparator 410, and frequency counter 414. Insome embodiments, some functionality can be implemented by a digitalcircuit such as a microcontroller 418.

The receiver tank circuit 402 can include the capacitor C1 and thesingle inductor L1 configured to measure the AC magnetic field along asingle axis. The single inductor L1 can be disposed in the wheel 202like the inductor 204 described with reference to FIG. 2. The inductorL1 can generate a voltage that is input to the receive input amplifier404 as a result of the time varying magnetic field generated by atransmit antenna (e.g., the buried cable 104). In some embodiments, thesingle inductor L1 can be the only inductor used to detect the VLFsignal from the buried cable 104. In some embodiments, the singleinductor L1 is arranged along one axis without a second inductorarranged along a different axis (e.g., at right angles to the firstaxis) for detecting the VLF signal.

The receive input amplifier 404 boosts the input waveform and provides aboosted waveform. An output of the receive input amplifier 404 is ACcoupled to a second receive amplifier 406, where the boosted waveform isfurther amplified. The second receive amplifier 406 provides anamplified waveform. The positive edge of the amplified waveform isclamped by capacitor C2 and diode D1.

In the example shown in FIG. 4A, the amplified waveform is fed to thewaveform feature identifier circuits 420 and 422. In variousembodiments, the amplified waveform can be fed to a single featureidentifier circuit (e.g., the circuit 420, with the circuit 422 notused) or to a plurality of greater than two feature identifier circuits.Each feature identifier circuit can have different component values(resistor and capacitor values) and be configured to detect differentwaveform features, operate under different noise conditions, detectdifferent features in frequencies, or operate with different sensitivitylevels. In some embodiments, the different feature identifier circuitscan have different circuit layouts. A logic controller can determine,based on the outputs of the waveform feature identifier circuits, whichdirection a cart is moving relative to the buried cable that isgenerating the time varying magnetic field. In the example shown in FIG.4A, the amplified waveform is fed to two feature identifier circuitsthat have similar layouts and different component values.

In a first feature identifier circuit 420, the amplified waveform is fedto a comparator 408. The comparator 408 is set to trip at a voltagebelow the positive edge of the amplified waveform, the voltage being setby resistors R1 and R2. Capacitor C3 holds the inverting input of thecomparator 408 at the average voltage seen at the junction of resistorsR1 and R2. A frequency counter 412 can count or determine timings of howfrequently the output of comparator 408 changes.

In the receiver 400 shown in FIG. 4A, the amplified waveform from asecond feature identifier circuit 422 is fed to a second comparator 410.The comparator 410 is set to trip at a voltage below the positive edgeof the amplified waveform, the voltage being set by resistors R3 and R4.Capacitor C4 holds the inverting input of the comparator 410 at theaverage voltage seen at the junction of resistors R3 and R4. A frequencycounter 414 can count or determine the timings of how frequently theoutput of comparator 410 changes.

A logic controller 416 can, based on the outputs of the frequencycounters, determine whether a cart is moving in one direction oranother. For example, based on the waveform shown in FIG. 3A, thefrequency or timings of the waveform features (such as maxima or minima,or whether voltages (or currents) pass certain thresholds or have slopesthat pass certain thresholds) can be detected in order to distinguish,for example, the waveform 306 from the waveform 308 and thus todetermine the relative direction of the wheel (and the cart) to theburied cable. The logic controller 416 can comprise a digital signalprocessor (DSP), hardware microprocessor, programmable logic device(PLD), application-specific integrated circuit (ASIC), or other type ofhardware logic circuitry.

FIG. 4B is a block diagram of another embodiment of a single inductordirection-detector receiver 430 that shares similar features to thereceiver 400 shown in FIG. 4A. In FIG. 4B, the receiver tank circuit 432includes a single inductor L1, capacitors C4 and C1, and a field effecttransistor (FET) switch. The receiver 430 also includes one waveformfeature identifier circuit 434 having a comparator similar to thecomparators 408, 410 in FIG. 4A.

The center frequency of the resonant receive tank circuit 432 is loweredby a factor f when the FET switch is closed, because the capacitance ofthe tank 432 becomes C1+C4 rather than C1. This allows upper and lowerfrequency components of the received waveform to be equally spaced (inratio) above and below the resonant frequency of the tank circuit. Thiscircuit design may reduce or minimize phase shift variations due to tankcircuit component variations. For a case where the frequencies in thewaveform are in a 3:2 ratio, the factor f can be 1/√{square root over(⅔)}, which can be achieved when C4=C1/2. For cases where thefrequencies in the waveform are in a 2:1 ratio, the factor f can be1/√{square root over (2)}, which can be achieved when C4=C1. Forimplementations using an asymmetric waveform with a first frequency atthe VLF frequency and a second frequency at a fraction (e.g., ½, ⅔,etc.) of the VLF frequency, the FET can be configured to pull down thefrequency by a factor equal to the square root of this fraction (e.g.,so that this frequency is the geometric mean of the two signalfrequencies).

In some installations (e.g., retail stores), there may be a mix ofsignal transmitters with some transmitters transmitting adirection-crossing sensitive asymmetric signal (e.g., at a parking lotperimeter) and some transmitters (which may be less complicated andthereby less expensive) transmitting non-direction-crossing sensitivesignals (e.g., at beacons used in a cart containment system). Thus,there can be an advantage in providing a wheel receiver which is able tobe programmed to work with both types of transmitters. Thus, asdescribed with reference to FIG. 4B, the capacitor C4 and the FET switchprovides the receiver 430 with selectable program modes, with a firstmode that is backward compatible with legacy non-direction-crossingsensitive transmitters (e.g., having a center frequency at 8 kHz and lowfractional bandwidth) and with a second mode that is compatible with thedirection-crossing-sensitive asymmetric signals (e.g., with a centerfrequency at a geometric mean of high and low signaling frequencies andmore fractional bandwidth). In other embodiments (e.g., the receiver 400of FIG. 4A), the FET switch is not used (e.g., because selectableprogram modes are not needed or backward compatibility with legacysystems is not needed), and the value of C1 in the receiver 400 can beselected to be equal to the value of C1+C4 in the receiver 430.

FIG. 4C is a block diagram of another example of a single inductordirection-detector receiver 460 that shares features of the receivers400, 430 shown in FIGS. 4A and 4B. The receiver 460 can be used todetect an asymmetric ramp waveform (see, e.g., an example shown in FIG.6). In FIG. 4C, the resonant receive tank circuit 462 includes thesingle inductor L1, a capacitor C1, and an FET switch. FIG. 4C alsoincludes one waveform identifier circuit 464.

The FET switch opens when a ramp is being transmitted. This allows thehigh slew rate portion of the edge to pass through the inductor L1without as much ringing that might result if capacitor C4 of thereceiver 430 were in the circuit. The receiver 460 includes an edgedetector (e.g., a Schmitt trigger) as part of the waveform featureidentifier circuit 464.

FIG. 5A shows an example schematic of a single inductor directiondetector receiver 500. In this example, the receiver 500 is configuredfor dual sine wave detection with a tank circuit frequency shifter withone waveform feature identifier circuit 434. Examples of a receiver thatcan detect an asymmetric (e.g., dual sine wave) signal are describedwith reference to FIGS. 3A and 3B. This receiver 500 generallycorresponds to the block diagram in FIG. 4B. Dual sine waves aredescribed for illustration but the receivers disclosed herein can beused to detect other asymmetric signals comprising, e.g., superpositionsof 3, 4, 5, 6, 10, 100, or more sinusoidal waves or other asymmetricsignals.

As described with reference to FIGS. 4A-4C, the receiver circuit 500uses a single inductor L1 to sense a single component of the magneticfield waveform generated by the current in a transmitter, such as theburied cable 104. The magnetic field (from the buried cable) induces avoltage on the inductor L1 which is part of the tank circuit 432 whichincludes capacitor C21, capacitor C20 (respectively corresponding to C1and C4 in FIG. 4B) and resistor R9. The resistance value of resistor R9can be adjusted in order to change the fractional bandwidth of the tankcircuit 432. In this receiver, the capacitor C20 and resistor R18 willlower the resonant frequency and change the fractional bandwidth when anFET Q6 is tuned on. When Q6 is turned on, the center frequency of thetank circuit is lowered by

$\frac{1}{\sqrt{2}}$the factor f, which as described above can be the geometric mean of thefrequencies of the asymmetric waveform (e.g., √{square root over (½)},√{square root over (⅔)}, etc.). This allows upper and lower frequencycomponents of the incoming signal to be equally spaced above and belowthe resonant frequency of the tank circuit. This may reduce or minimizephase shift variations due to tank circuit component variations.

FIG. 5B shows an example schematic of another single inductor directiondetector receiver 530. The circuit in FIG. 5B can be configured togenerate outputs based on a plurality of (in this case, four) differentgain values. A first gain control circuit includes resistor R2 andtransistor Q8, where the gain can be set based at least in part onresistor R2, and an output can be taken from the transistor Q8. A secondgain control circuit includes resistor R20 and transistor Q10, where thegain can be set based at least in part on R20, and an output can betaken from the transistor Q10. For outputs taken from either transistorQ8 or Q10, the gain can also be affected by the configuration of switchQ6. Accordingly, at least four different gains are possible, and othercircuits can have additional configurations in the tank circuit 432 ormore outputs, further increasing the number of selectable gains. Othersystems can use other variable components, such as varactors, variableresistors, etc. to adjust the gain. A control loop can be implemented byfirmware, logic 416, or other system to configured select among the gainconfigurations and sufficiently boost the detected signal withoutsaturating the output stage. The single inductor direction detectorreceiver 530 can include a filter, such as the filter including resistorR21 and capacitor C26. Resistor R21 and capacitor C26 can have a commonmode that is coupled to an input of the comparator U2, such as thenon-inverting input.

The schematics shown in FIGS. 5A and 5B are substantially similar, butbecause of the different outputs of the comparator (into a frequencycounter or into an edge detector, respectively), some component values(e.g., for capacitors, resistors, diodes, and active components) can beoptimized differently.

A schematic corresponding to FIG. 4A, which features multiple waveformfeature identifier circuits that each include a comparator, can besimilar to the schematic 500. In comparison to the schematic 500, aschematic implementation corresponding to FIG. 4A can duplicatecomponents R7, R14, and C13 (which form a low pass filter) and replacethe U2 comparator device with a multi-channel (e.g., dual or quadchannel) comparator device. In some embodiments, the dual channelschematic may or may not also duplicate diode D4 and resistor R17.

FIG. 6 is a graph that shows an example of a 500 Hz saw tooth waveform606 that is bandwidth limited to 8 kHz. The y-axis 602 indicates avoltage in mV. The x-axis indicates time in ms. The waveform 606 has apositive slope on the rising portions of the waveform that is steeper(e.g., absolute magnitude of the slope is larger) than the negativeslope on the falling portions of the waveform, and the waveform 606 hasat least one asymmetric component.

The sawtooth waveform can produce a pulse at the receiver proportionalto the edge rate of the faster edge (e.g., the steeper edge). This canallow the receiver to detect the polarity of the signal by detecting thedirection of the received pulse. Any of the receivers described herein(e.g., FIGS. 4A-5B) can be used to detect the polarity of the examplesaw tooth waveform of FIG. 6.

Tilt Detection and Response

A person pushing a shopping cart may try to evade detection of theirattempted theft of the cart by tilting the cart to avoid triggering alocking or braking wheel. For example, the person may tilt the cart nearthe perimeter so that the locking or braking wheel is raised well abovethe surface so as to reduce the likelihood that the receivers describedherein that are disposed in the wheel will detect the electromagneticsignal from the buried cable. By tilting the cart and raising the wheel,the receiver circuits are moved farther away from the buried cable asthe tilted cart is pushed across the containment boundary, which reducesthe magnitude of the fluctuating VLF asymmetric magnetic field at theposition of the wheel (due to the roughly 1/r falloff of the magneticfield), thereby reducing the likelihood that the magnetic field isdetected. As discussed with respect to FIG. 2, the inductor 204 can beoriented on an axis that that is substantially parallel to the directionof the magnetic field vector {right arrow over (B)} generated by theburied cable 104 such that a voltage is formed across the inductor 204.Operation of the circuits for determining the direction of the cart canbe affected by the angle θ of the inductor 204 relative to the expecteddirection of the magnetic field (see FIG. 2). Changes in the anglecaused by tilting the cart can reduce the sensitivity of the inductor204. At certain inductor angles, the voltage across the inductor 204 canbe too low to be reliably detected, which may allow the person to tiltthe cart and pass over the buried cable 104 without triggering theanti-theft mechanism.

Accordingly, some embodiments of the wheel comprise an angle sensor,gyroscope, accelerometer, or other angle detection device 230 that canbe used to detect whether or not the cart has been tilted. In someembodiments, an angle detection device 230 is alternatively (oradditionally) included in the cart (e.g., in the frame). In the eventthat a tilt is detected (e.g., the tilt angle measured by the angledetection device exceeds a threshold angle, e.g., 10 degrees, 20degrees, 30 degrees, 40 degrees, or more) and proximity to the buriedcable is detected (e.g., the VLF signal from the cable 104 is detected),a corrective action can be taken such as, e.g., activating theanti-theft mechanism of the cart or sounding an alarm. The correctiveaction can be taken regardless of which direction of travel isdetermined or regardless of whether or not the direction of travel canbe detected at all.

In some embodiments, the angle detection device 230 comprises anaccelerometer that can determine the direction of the Earth'sgravitational acceleration. For an untilted cart, the acceleration isalong a vertical axis to the surface and can be measured as such by theaccelerometer. However, if the cart is tilted, the accelerometer willalso be tilted, and the measured vertical component of the gravitationalacceleration will change, permitting determination of the tilt angle(e.g., the measured value will be reduced by the cosine of the tiltangle). Another embodiment of the angle detection device 230 comprises alow frequency (e.g., less than 100 Hz) magnetometer configured to detectthe local vertical component of the Earth's magnetic field. As with theaccelerometer, tilting the cart (and the magnetometer) leads tocorresponding changes in the measured vertical geomagnetic component,from which the tilt angle can be determined.

Although the angle detection device 230 can be disposed in the wheel, inother embodiments, the angle detection device is (additionally oralternatively) disposed elsewhere in the cart, for example, in the frameor handlebars of the cart.

Example Cart Direction Detection Techniques with Non-Saturated Waveforms

FIG. 7A is a graph 700 that shows example waveforms that can be used todetermine the direction of a cart. FIG. 700 includes a y-axis 702, anx-axis 704, a first curve 706, a second curve 708, and a third curve710. The y-axis indicates a normalized scale for each curve in anappropriate unit (A/m for curve 706, V for curve 708, and V for curve710). The x-axis indicates time in milliseconds.

The first curve 706 indicates a vector component of a magnetic fieldthat is detected by the single inductor (e.g., L1) of a cart on a firstside of a cable (and can be moving toward the cable in a firstdirection). The curve 706 can be the component of the magnetic fieldthat is parallel to the axis of the single inductor. The first curve canbe the sum of two sine waves one octave apart, at 7.776 kHz and 3.888kHz, e.g., the ratio of frequencies is 2:1 (similar to the waveform ofthe current in FIG. 3B), where the amplitude of the 7.776 kHz componentis 1.5 times the amplitude of the 3.888 kHz component. In the examplegraph 700, the normalized scale for the first curve 706 is +/−0.02 A/m,but the normalized scale can be different in various embodiments.

The first curve 706 can be linearly related to the current in a buriedcable (e.g., the generated magnetic field is proportional to cablecurrent according to the Biot-Savart law). The first curve 706 can, insome embodiments, have a zero DC component. The first curve can also beasymmetric about a zero in the y-axis. The peak amplitude of the firstcurve 706 in one direction (e.g., the positive direction) is about 33%greater than the peak amplitude in the other direction. In someembodiments, the difference in peak amplitudes, slopes, or othercharacteristics can be about 25% to 50% greater in one direction versusthe other direction.

A second curve 708 can be a voltage formed across the single inductor(e.g., the inductor L1 in FIG. 4A, FIG. 4B, or FIG. 4C) in a tankcircuit (e.g., tank circuit 402, 430, or 462). The voltage can beprovided to an amplifier (e.g., amplifier 404 in FIG. 4A). In FIG. 5A,FIG. 5B, the voltage can be provided at the node between inductor L1,capacitor C21, resistor R2, diode D2, and transistor Q4. In the examplegraph 700, the normalized scale for the second curve 708 is +/−100 mV,but the normalized scale can be different in various embodiments.

The third curve 710 can be an output (8 kHz_RCVR_DATA in FIG. 5A andFIG. 5B) of a comparator (e.g., the comparator 408 or 410). In theexample graph 700, the normalized scale for the third curve 710 can be 0to VDD (which can be 1.8 V in some embodiments) or any digital output,but the normalized scale can be different in various embodiments.

FIG. 7B is a graph 750 that shows example waveforms that can be used todetermine the direction of a cart. FIG. 750 includes a y-axis 752, anx-axis 754, a first curve 756, a second curve 758, and a third curve760. The solid, dashed, and dot-dashed curves in FIG. 7B correspond tothe same curves as described with respect to FIG. 7A, except in in FIG.7B, the cart is on a second side of a cable and can be moving toward thecable from a second direction.

FIGS. 7C and 7D are generally similar to FIGS. 7A and 7B, respectively,but at different points in the circuit. In FIG. 7C, the solid curve 706is the negative input to the comparator, the dashed curve 708 is thepositive input to the comparator (e.g., represents a low pass filteredversion of the amplifier output), and the dot-dashed curve 710 is thecomparator output.

In comparing FIG. 7A and FIG. 7B (or FIGS. 7C and 7D), the comparatoroutput toggles once per 257 microsecond cycle (1/3.888 kHz) when themagnetic field waveform orientation is positive (e.g., when the cart ison the first side of the cable as shown in FIG. 7A), and the comparatortoggles twice per 257 microsecond cycle when the magnetic field waveformorientation is negative (e.g., when the cart is on the second side ofthe cable as shown in FIG. 7B). Thus, by counting the frequency withwhich the comparator output toggles (3.888 kHz or 7.776 kHz) it can bedetermine whether the magnetic field is oriented positively ornegatively, and thus which side of the buried cable that the cart is on.In some embodiments, the output of the comparator can be delayed (e.g.,by about 8 microseconds) due to circuit delays.

There can be an error band around the two expected frequencies to addsome noise tolerance. For example, if the measured comparator togglefrequency is within 10% of 3.888 kHz, the cart can be determined to beon the first side of the buried cable, and if the measured comparatortoggle frequency is within 10% of 7.776 kHz, the cart can be determinedto be on the second side of the buried cable. If the comparator togglerate is not within either of those two frequency ranges, the system canassume that the signal is too noisy to be trusted and can keep measuringuntil a valid toggle rate is detected.

Example Cart Direction Detection Techniques Under Saturation Conditions

The waveforms in FIGS. 7A-7D show examples where the receiver isunsaturated. Under certain conditions, such as if the inductor ispositioned very close to the buried cable (e.g., when the wheel crossesthe containment boundary), then the magnetic field strength can becomerelatively stronger and the receiver may saturate.

FIG. 8A is a graph 800 that shows example waveforms under saturationconditions that can be used to determine the direction of a cart. Thegraph 800 includes a y-axis 802, an x-axis 804, a first curve 806, asecond curve 808, and a third curve 810 that correspond to the axis andcurves discussed with respect to FIG. 7A.

FIG. 8B is a graph 850 that shows example waveforms under saturationconditions that can be used to determine the direction of a cart. Thegraph 850 includes a y-axis 852, an x-axis 854, a first curve 856, asecond curve 858, and a third curve 860 that correspond to the axis andcurves discussed with respect to FIG. 7B.

Under the saturation conditions of FIG. 8A and FIG. 8B, the receiver issaturated and no longer linear. This can happen, for example, if thestrength of the magnetic field increases (e.g., doubles, triples,exceeds a threshold such as 0.04 A/m) as a receiver approaches theburied cable. In some embodiments, firmware, feedback, or other controlsystem can adjust a gain to keep the receiver in the linear range.Nonetheless, a receiver can still saturate even with adjustments togain.

In the example saturation conditions shown in both FIG. 8A and FIG. 8B,the comparator toggles at the same rate, once per 257 microsecond cycle.However, the duty cycle of the comparator output is different and can beused to determine which side of a cable that the cart is on orapproaching from. In FIG. 8A, the comparator output signal 810 has ahigh duty cycle of about 72% when the cart is on the first side of theburied cable. In FIG. 8B, the comparator output signal 860 has a highduty cycle of about 45% when the cart is on the second side of theburied cable.

Some microcontrollers have hardware which can perform automatic dutycycle measurement on a specified input. The microcontrollers can, basedat least in part on the duty cycle, determine which side of the buriedcable that a cart is on. In some embodiments, the duty cycle measurementcan be performed by firmware or other circuits by timing the rising andfalling edges and then calculating the duty cycle.

Example Cart Direction Detection Techniques

With respect to FIG. 4A and FIG. 7A-7D, a tank circuit 402 detects asingle component of a magnetic field (e.g., the component of thefluctuating magnetic field that is parallel to the single inductor L1),which can be amplified by the second receive amplifier 406 to generatean amplified signal 706 or 756. The amplified signal 706 or 756 can becompared to a low pass filtered version (708, 758, respectively) of thatsame signal 706 or 756. An example low pass filter in FIG. 4A includesresistor R1, resistor R2, and capacitor C3. The combination of the lowpass filter and comparator can be referred to as a data slicer becauseit slices the continuous analog amplifier 406 output into discrete timechunks which are individual bits.

In some designs, it can be less cost effective and less power efficientto implement the low pass filter to make the filter characteristics suchas time constant and voltage offset dynamic. For example, digitizing thesignal and using a digital signal processor (DSP) can consume too muchpower for a battery powered wheel if a design goal includes having anactive power consumption of a value that is much less than 1 mW. Theexample circuits shown in FIG. 5A and FIG. 5B consume power on the orderof about 30 to 100 μW when the line is being driven, and consume poweron the order of 20 μW when the line is not being driven.

In some embodiments, component values for the low pass filter can beselected to achieve adequate compromise performance characteristicsacross various scenarios. However, in some situations where theamplitude of the magnetic field component parallel to the inductor axis(e.g., the maximum of the dot product of {right arrow over (B)} with aunit vector parallel to the inductor axis) is changing relativelyquickly (e.g., at least 35%, at least 25%, at least 45% decline inamplitude over about two cycles or less) to the waveform repeat time(e.g., 257 microseconds in FIG. 8A and FIG. 8B), a situation can occurwhere a second trip point of the comparator does not always trip. FIG.7D shows examples of a first trip point 761 and a second trip point 762.The first trip point 761 can happen at the same time or either side ofthe buried cable, whereas the second trip point 762 typically onlyhappens on the negative side of the cable. When the second trip point ofthe comparator does not trip, the comparator output can appearsuperficially similar to the saturated receiver described with referenceto FIG. 8A and FIG. 8B.

The amplitude of the magnetic field component parallel to the inductoraxis can change relatively quickly under certain conditions. Forexample, the change can occur when a receiver approaches a locationwhere a buried cable 104 makes a sharp bend, such as a right angle. Morepractical, large-scale cable burying tools can cut grooves into theground in a straight line for burying a cable. To avoid sharp bends, two45-degree beveled corners can be spread several inches to several feetapart instead of having a right angle. As another example, the changecan occur when the receiving inductor travels a path that is nearlyparallel to and nearly directly above the buried cable 104. There can belittle to no vertical component of the magnetic field {right arrow over(B)}. When the magnetic field {right arrow over (B)} is imposed on aninductor of a finite radius under such conditions, the resulting EMFinduced in the inductor can be very noisy. As another example, thechange can occur due to high levels of background noise, such as if acart travels close to a high current power line in a buried conduit.

In some embodiments, DSPs can dynamically change filter settings toadjust for these conditions. In some embodiments where power constraintsmake DSP impractical, a circuit can implement two different fixed lowpass filters with different characteristics, and two differentcomparator outputs can be generated (e.g., as shown in FIG. 4B). Basedon differences in the outputs of the two different comparators, theposition of a cart with respect to the buried cable 104 can bedetermined with greater accuracy.

Different algorithms can be used to determine the position of the cartbased on the outputs of different comparators. In an example, theoutputs of two different comparators with low pass filters are the same,then the cart can be determined to be on a first side of the buriedcable 104. If either of the outputs of the two comparators is fairlyconsistently counting two transitions per cycle (e.g., as shown in FIG.7D), then the cart can be determined to be on the second side of theburied cable 104.

Additional Embodiments

In some cases it may be desirable to encode additional information inthe signal beyond which direction in which the wheel is crossing theburied line. For example, there may be multiple doors with a buried linemarking the door. It may be useful to also encode which door of themultiple doors the wheel is passing through, along with the direction.

One way to achieve encode the additional information (e.g., which door)is to consider an interval of fixed duration (e.g., a bit cell) of lowfrequency (e.g., 3.888 kHz) toggling to be one value (e.g., “1”) in asignaling code, with an interval of high frequency (e.g., 7.776 kHz)toggling to be an alternate value (e.g., “−1”). A variety of codingschemes can handle the ambiguity of whether a given bit cell was sent asa “1” with the receiver on the positive side of the buried cable or as a“−1” with the receiver on the negative side.

In some embodiments, an encoding scheme can include an inversiontolerant duobinary line coding scheme such as Alternate Mark Inversionor a differential ternary code such as MLT-3.

In some embodiments, an encoding scheme can include an asynchronous codewith a start bit of known polarity. If a start bit of reversed polarityis found, then a direction-detector can determined that the cart is onthe negative side of the buried line and invert all the received bits.

In some embodiments, an encoding scheme can include sending codewordsout of a set of codewords such that the Hamming distance between validcodewords is at least two, and that the inversion of any valid codewordis not a valid codeword. Both the received bit string and the inversionof the received bit string can be tested, and the string with the lowestHamming distance to a valid codeword can be selected.

Although certain embodiments of the single inductor direction detectorhave been described as being disposed within the wheel of the cart, thisis for illustration and is not a requirement. In other embodiments, someor all of the receiver components can be disposed in other portions ofthe cart. As one example, the single inductor (e.g., the resonant tankcircuit) can be disposed in a frame of the cart, e.g., a lower plasticportion of the frame where the inductor is not shielded from thecontainment signal by metal tubing. Other variations are possible.

Additional Example Embodiments

Additional example embodiments can include the following.

In a first embodiment, a system to detect whether an object crosses aboundary in a direction, the system comprising: a cable surrounding acontainment area and defining a boundary of the containment area; atransmitter electrically connected to the cable and configured totransmit a radio frequency (RF) containment signal to the cable, thecontainment signal comprising an asymmetric component wherein amagnitude of a positive slope of the signal is different from amagnitude of a negative slope of the signal; and a receiver configuredto detect the RF containment signal, the receiver comprising a resonanttank circuit having a single inductor, wherein the system determines adirection of the object relative to the boundary of the containment areaby comparing an electromotive force (EMF) induced in the single inductorby a first features of the signal with an EMF induced in the singleinductor by a second feature of the signal.

In a second embodiment, the system of embodiment 1, wherein the objectcomprises a wheel of a human-propelled cart.

In a third embodiment, the system of embodiment 2, wherein the cartcomprises a shopping cart.

In a fourth embodiment, the system of any one of embodiments 1-3,wherein the containment area comprises a parking lot of a retail store.

In a fifth embodiment, the system of any one of embodiments 2-5, whereinthe receiver is disposed in the wheel.

In a sixth embodiment, the system of any one of embodiments 1-5, whereinthe RF containment signal comprises a very low frequency (VLF) signal.

In a seventh embodiment, the system of embodiment 6, wherein the VLFsignal is below about 9 kHz.

In an eighth embodiment, the system of any one of embodiments 1-7,wherein an inductance of the single inductor is in a range from 1 mH to50 mH.

In a ninth embodiment, the system of any one of embodiments 1-8, whereinthe RF containment signal comprises a first frequency and a secondfrequency, and a ratio of the second frequency to the first frequency isabout 2:1 or about 3:2.

In a 10th embodiment, the system of any one of embodiments 1-9, whereinthe receiver comprises a frequency counter or an edge detector.

In an 11th embodiment, the system of any one of embodiments 1-10,wherein, when installed, the cable is buried below a surface of thecontainment area.

In a 12th embodiment, the system of any one of embodiments 1-11, whereinthe single inductor is disposed at an angle relative to a vertical to asurface of the containment area. The angle can be about zero degrees,about 90 degrees, or in a range from about 5 degrees to about 25degrees.

In a 13th embodiment, the system of any one of embodiments 1-12, whereinthe RF containment signal comprises a bandwidth limited saw tooth ramp.

In a 14th embodiment, the system of any one of embodiments 1-13, whereinthe receiver further comprises a receive input amplifier.

In a 15th embodiment, the system of any one of embodiments 1-14, whereinthe receiver comprises a clamp circuit.

In a 16th embodiment, the system of any one of embodiments 1-15, whereinthe receiver comprises a comparator.

In a 17th embodiment, a receiver configured to detect an RF containmentsignal near a boundary of a containment area, the receiver comprising: aresonant tank circuit having a single inductor, wherein the systemdetermines a direction of the receiver relative to the boundary of thecontainment area by comparing an electromotive force (EMF) induced inthe single inductor by the positive slope of the signal with an EMFinduced in the single inductor by the negative slope of the signal.

In an 18th embodiment, a wheel for a human-propelled cart, the wheelcomprising the receiver of embodiment 17.

In a 19th embodiment, the wheel of embodiment 18 further comprising abrake configured to inhibit motion of the cart when actuated.

In a 20th embodiment, a containment system comprising the receiver ofembodiment 17 or the wheel of embodiment 18 or 19, a cable, and a radiofrequency transmitter configured to be electrically connected to thecable.

In a 21st embodiment, a system to detect whether a human-propelled carthaving a wheel crosses a boundary, the system comprising: a cablesurrounding a containment area and defining a boundary of thecontainment area; a transmitter electrically connected to the cable andconfigured to transmit a radio frequency (RF) containment signal to thecable, the containment signal comprising an asymmetric, fluctuatingcomponent, the cable thereby generating an asymmetric, fluctuatingmagnetic field having three components; and a wheel comprising areceiver configured to detect the RF containment signal, the receivercomprising: a resonant tank circuit having a single inductor configuredto measure a single component of the three components of the asymmetric,fluctuating magnetic field, and a hardware processor programmed todetermine a direction of the cart relative to the boundary of thecontainment area based at least in part on the measured single componentof the three components of the asymmetric, fluctuating magnetic field.

In a 22nd embodiment, the system of embodiment 1, wherein thehuman-propelled cart comprises a shopping cart.

In a 23rd embodiment, the system of embodiment 21 or 22, wherein theasymmetric fluctuating component of the containment signal is invariantunder inversion and phase shifting.

In a 24th embodiment, the system of any one of embodiments 21 to 23,wherein the asymmetric fluctuating component of the containment signalcomprises a first sinusoidal component having a first frequency and asecond sinusoidal component having a second frequency less than thefirst frequency.

In a 25th embodiment, the system of embodiment 24, wherein a ratio ofthe second frequency to the first frequency is 1/2 or 2/3.

In a 26th embodiment, the system of embodiment 24 or 25, wherein thefirst frequency and the second frequency are less than 9 kHz.

In a 27th embodiment, the system of any one of embodiments 21 to 26,wherein the hardware processor is programmed to determine the directionof the cart relative to the boundary of the containment area byidentifying a plurality of features of the asymmetric fluctuatingcomponent of the containment signal during a time period that is lessthan a period of the asymmetric fluctuating component.

In a 28th embodiment, the system of any one of embodiments 21 to 27,wherein the receiver comprises a comparator configured to compare asignal representative of the measured single component of theasymmetric, fluctuating magnetic field with a low-pass filteredrepresentation of the measured single component of the asymmetric,fluctuating magnetic field.

In a 29th embodiment, the system of embodiment 28, wherein the hardwareprocessor is programmed to determine the direction of the cart relativeto the boundary of the containment area based at least in part on afrequency at which the comparator output toggles.

In a 30th embodiment, the system of embodiment 28 or 29, wherein thehardware processor is programmed to determine the direction of the cartrelative to the boundary of the containment area based at least in parton a duty cycle of output from the comparator.

In the 31st embodiment, the system of any one of embodiments 21 to 30,wherein the resonant tank circuit comprises a switch configured toswitch between a first resonant frequency and a second resonantfrequency different from the first resonant frequency.

In a 32nd embodiment, the system of any one of embodiments 21 to 31,wherein the receiver comprises a plurality of gain control circuitsconfigured to boost the measured single component of the asymmetric,fluctuating magnetic field without saturating an output stage of thereceiver.

In a 33rd embodiment, the system of any one of embodiments 21 to 32,wherein the single inductor is disposed on a non-rotating chassis of thewheel.

In a 34th embodiment, the system of embodiment 33, wherein the singleinductor has an axis that makes a non-zero angle with respect to avertical direction.

In a 35th embodiment, the system of any one of embodiments 21 to 34,wherein the cart comprises an anti-theft system, and the system isfurther configured to trigger the anti-theft system in response todetermining the cart crosses the boundary in a first direction and tonot trigger the anti-theft system in response to determining the cartcrosses the boundary in a second direction opposite to the firstdirection.

In a 36th embodiment, the system of embodiment 35, wherein the wheelcomprises a brake configured to inhibit motion of the cart when theanti-theft system is triggered.

In a 37th embodiment, the system of embodiment 35 or 36, wherein thewheel or the cart comprises an angle detection device configured todetermine a tilt angle of the cart, and the system is further configuredto trigger the anti-theft system in response to determining the tiltangle exceeds a threshold and the receiver has measured the containmentsignal.

In a 38th embodiment, a receiver configured to detect a radio frequency(RF) containment signal near a boundary of a containment area, thereceiver comprising: a resonant tank circuit having a single inductioncircuit configured to detect a single component of a magnetic fieldassociated with the RF containment signal; a first comparator configuredto compare a signal associated with the detected single component of themagnetic field against a first low-pass filtered representation of thesignal; and a logic control circuit configured to determine a positionof the receiver relative to the boundary of the containment area basedat least in part on a first output from the first comparator.

In a 39th embodiment, the receiver of embodiment 38, further comprisinga frequency counter configured to receive the output from thecomparator.

In a 40th embodiment, the receiver of embodiment 38 or 39, furthercomprising an edge detector configured to receive the output from thecomparator.

In a 41st embodiment, the receiver of any one of embodiments 38 to 40,wherein: the receiver comprises a second comparator configured tocompare the signal associated with the detected single component of themagnetic field against a second low-pass filtered representation of thesignal, and the logic control circuit is configured to determine theposition of the receiver relative to the boundary of the containmentarea based at least in part on the first output from the firstcomparator and the second output from the second comparator.

In a 42nd embodiment, the receiver of any one of embodiments 38 to 41,wherein the logic control circuit is configured to determine theposition of the receiver relative to the boundary of the containmentarea based at least in part on a duty cycle of the first output from thefirst comparator.

In a 43rd embodiment, the receiver of any one of embodiments 38 to 42,wherein the RF containment signal comprises a first sinusoidal componenthaving a first frequency and a second sinusoidal component having asecond frequency less than the first frequency.

In a 44th embodiment, the receiver of embodiment 43, wherein a ratio ofthe second frequency to the first frequency is 1/2 or 2/3.

In a 45th embodiment, the receiver of embodiment 43 or 44, wherein thefirst frequency and the second frequency are less than about 9 kHz.

In a 46th embodiment, a wheel for a human-propelled cart, the wheelcomprising the receiver of any one of embodiments 38 to 45.

In a 47th embodiment, the wheel of embodiment 46 further comprising abrake configured to inhibit motion of the wheel when the brake istriggered.

Additional Considerations

Certain processing steps or acts of the methods disclosed herein may beimplemented in hardware, software, or firmware, which may be executed byone or more general and/or special purpose computers, processors, orcontrollers, including one or more floating point gate arrays (FPGAs),programmable logic devices (PLDs), application specific integratedcircuits (ASICs), and/or any other suitable processing device. Incertain embodiments, one or more functions provided by a controller maybe implemented as software, instructions, logic, and/or modulesexecutable by one or more hardware processing devices. In someembodiments, the software, instructions, logic, and/or modules may bestored on computer-readable media including non-transitory storage mediaimplemented on a physical storage device and/or communication media thatfacilitates transfer of information. In various embodiments, some or allof the steps or acts of the disclosed methods or controllerfunctionality may be performed automatically by one or more processingdevices. Many variations are possible.

Conditional language used herein, such as, among others, “can,” “could,”“might,” “may,” “e.g.,” and the like, unless specifically statedotherwise, or otherwise understood within the context as used, isgenerally intended to convey that certain embodiments include, whileother embodiments do not include, certain features, elements and/orsteps. Thus, such conditional language is not generally intended toimply that features, elements and/or steps are in any way required forone or more embodiments or that one or more embodiments necessarilyinclude logic for deciding, with or without author input or prompting,whether these features, elements and/or steps are included or are to beperformed in any particular embodiment. The terms “comprising,”“including,” “having,” and the like are synonymous and are usedinclusively, in an open-ended fashion, and do not exclude additionalelements, features, acts, operations, and so forth. Also, the term “or”is used in its inclusive sense (and not in its exclusive sense) so thatwhen used, for example, to connect a list of elements, the term “or”means one, some, or all of the elements in the list. As used herein, aphrase referring to “at least one of” a list of items refers to anycombination of those items, including single members. As an example, “atleast one of a, b, or c” (or “at least one of a, b, and c”) is intendedto cover: a, b, c, a-b, a-c, b-c, and a-b-c. In addition, the articles“a,” “an”, and “the” as used in this application and the appended claimsare to be construed to mean “one or more” or “at least one” unlessspecified otherwise.

The example experiments, experimental data, tables, graphs, plots,photographs, figures, and processing and/or operating parameters (e.g.,values and/or ranges) described herein are intended to be illustrativeof operating conditions of the disclosed systems and methods and are notintended to limit the scope of the operating conditions for variousembodiments of the methods and systems disclosed herein. Additionally,the experiments, experimental data, calculated data, tables, graphs,plots, photographs, figures, and other data disclosed herein demonstratevarious regimes in which embodiments of the disclosed systems andmethods may operate effectively to produce one or more desired results.Such operating regimes and desired results are not limited solely tospecific values of operating parameters, conditions, or results shown,for example, in a table, graph, plot, figure, or photograph, but alsoinclude suitable ranges including or spanning these specific values.Accordingly, the values disclosed herein include the range of valuesbetween any of the values listed or shown in the tables, graphs, plots,figures, photographs, etc. Additionally, the values disclosed hereininclude the range of values above or below any of the values listed orshown in the tables, graphs, plots, figures, photographs, etc. as mightbe demonstrated by other values listed or shown in the tables, graphs,plots, figures, photographs, etc. Also, although the data disclosedherein may establish one or more effective operating ranges and/or oneor more desired results for certain embodiments, it is to be understoodthat not every embodiment need be operable in each such operating rangeor need produce each such desired result. Further, other embodiments ofthe disclosed systems and methods may operate in other operating regimesand/or produce other results than shown and described with reference tothe example experiments, experimental data, tables, graphs, plots,photographs, figures, and other data herein. Also, for various valuesdisclosed herein, relative terms “about”, “nearly”, “approximately”,“substantially”, and the like may be used. In general, unless indicatedotherwise, relative terms mean within ±20%, within ±15%, within ±10%,within ±5%, depending on the embodiment.

Thus, while only certain embodiments have been specifically describedherein, it will be apparent that numerous modifications may be madethereto without departing from the spirit and scope of the disclosure.Further, acronyms are used merely to enhance the readability of thespecification and claims. It should be noted that these acronyms are notintended to lessen the generality of the terms used and they should notbe construed to restrict the scope of the claims to the embodimentsdescribed therein.

What is claimed is:
 1. A system to detect whether a human-propelled carthaving a wheel crosses a boundary, the system comprising: a cablesurrounding a containment area and defining a boundary of thecontainment area; a transmitter electrically connected to the cable andconfigured to transmit a radio frequency (RF) containment signal to thecable, the containment signal comprising an asymmetric, fluctuatingcomponent, the cable thereby generating an asymmetric, fluctuatingmagnetic field having three components; and the wheel comprising areceiver configured to detect the RF containment signal, the receivercomprising: a resonant tank circuit having a single inductor configuredto measure a single component of the three components of the asymmetric,fluctuating magnetic field, and a hardware processor programmed todetermine a direction of the cart relative to the boundary of thecontainment area based at least in part on the measured single componentof the three components of the asymmetric, fluctuating magnetic field.2. The system of claim 1, wherein the human-propelled cart comprises ashopping cart.
 3. The system of claim 1, wherein the asymmetricfluctuating component of the containment signal is invariant underinversion and phase shifting.
 4. The system of claim 1, wherein theasymmetric fluctuating component of the containment signal comprises afirst sinusoidal component having a first frequency and a secondsinusoidal component having a second frequency less than the firstfrequency.
 5. The system of claim 4, wherein a ratio of the secondfrequency to the first frequency is 1/2 or 2/3.
 6. The system of claim4, wherein the first frequency and the second frequency are less than 9kHz.
 7. The system of claim 1, wherein the hardware processor isprogrammed to determine the direction of the cart relative to theboundary of the containment area by identifying a plurality of featuresof the asymmetric fluctuating component of the containment signal duringa time period that is less than a period of the asymmetric fluctuatingcomponent.
 8. The system of claim 1, wherein the receiver comprises acomparator configured to compare a signal representative of the measuredsingle component of the asymmetric, fluctuating magnetic field with alow-pass filtered representation of the measured single component of theasymmetric, fluctuating magnetic field.
 9. The system of claim 8,wherein the hardware processor is programmed to determine the directionof the cart relative to the boundary of the containment area based atleast in part on a frequency at which the comparator output toggles. 10.The system of claim 8, wherein the hardware processor is programmed todetermine the direction of the cart relative to the boundary of thecontainment area based at least in part on a duty cycle of output fromthe comparator.
 11. The system of claim 1, wherein the resonant tankcircuit comprises a switch configured to switch between a first resonantfrequency and a second resonant frequency different from the firstresonant frequency.
 12. The system of claim 1, wherein the receivercomprises a plurality of gain control circuits configured to boost themeasured single component of the asymmetric, fluctuating magnetic fieldwithout saturating an output stage of the receiver.
 13. The system ofclaim 1, wherein the single inductor is disposed on a non-rotatingchassis of the wheel.
 14. The system of claim 13, wherein the singleinductor has an axis that makes a non-zero angle with respect to avertical direction.
 15. The system of claim 1, wherein the cartcomprises an anti-theft system, and the system is further configured totrigger the anti-theft system in response to determining the cartcrosses the boundary in a first direction and to not trigger theanti-theft system in response to determining the cart crosses theboundary in a second direction opposite to the first direction.
 16. Thesystem of claim 15, wherein the wheel comprises a brake configured toinhibit motion of the cart when the anti-theft system is triggered. 17.The system of claim 15, wherein the wheel or the cart comprises an angledetection device configured to determine a tilt angle of the cart, andthe system is further configured to trigger the anti-theft system inresponse to determining the tilt angle exceeds a threshold and thereceiver has measured the containment signal.
 18. A wheel forhuman-propelled cart, the wheel comprising a receiver configured todetect a radio frequency (RF) containment signal near a boundary of acontainment area, the receiver comprising: a resonant tank circuithaving a single induction circuit configured to detect a singlecomponent of a magnetic field associated with the RF containment signal;a first comparator configured to compare a signal associated with thedetected single component of the magnetic field against a first low-passfiltered representation of the signal; and a logic control circuitconfigured to determine a position of the receiver relative to theboundary of the containment area based at least in part on a firstoutput from the first comparator.
 19. The wheel of claim 18, furthercomprising a frequency counter configured to receive the output from thecomparator.
 20. The wheel of claim 18, further comprising an edgedetector configured to receive the output from the comparator.
 21. Thewheel of claim 18, wherein: the receiver comprises a second comparatorconfigured to compare the signal associated with the detected singlecomponent of the magnetic field against a second low-pass filteredrepresentation of the signal, and the logic control circuit isconfigured to determine the position of the receiver relative to theboundary of the containment area based at least in part on the firstoutput from the first comparator and the second output from the secondcomparator.
 22. The wheel of claim 18, wherein the logic control circuitis configured to determine the position of the receiver relative to theboundary of the containment area based at least in part on a duty cycleof the first output from the first comparator.
 23. The wheel of claim18, wherein the RF containment signal comprises a first sinusoidalcomponent having a first frequency and a second sinusoidal componenthaving a second frequency less than the first frequency.
 24. The wheelof claim 23, wherein a ratio of the second frequency to the firstfrequency is 1/2 or 2/3.